This invention relates to a homogeneous, ruthenium catalyzed liquid phase process for vinyl ester synthesis. The process provides lower reaction temperatures and higher selectivity than prior zinc based acetylene routes and ruthenium based gas phase routes to vinyl esters.
The addition of carboxylic acids to acetylenically unsaturated compounds (i.e., alkynes) is known to be catalyzed by strong acids, Lewis acids, and electrophiles. The literature suggests that the preferred catalysts for vinylation reactions have been cadmium salts and zinc salts of the carboxylic acid in combination with a metal-containing Lewis acid. Gas phase reaction of acetylene and carboxylic acids has been accomplished in the presence of zinc and cadmium salts of the carboxylic acid, and mercury salts. Significant deficiencies in the use of this catalyst include toxicity concerns over the use of mercury and lack of selectivity and stability of zinc-based catalysts.
Ruthenium based catalysts have been found to promote the addition of carboxylic acids to alkynes and to produce alkenyl carboxylates. The reaction of carboxylic acids with acetylene (vinylation) to give vinyl esters is known to occur at a much slower rate and with lower selectivities to the desired vinyl esters. Various catalyst precursors have been studied to improve the rate of this reaction, including ruthenium carbonyl, bis(eta 5-cyclooctadienyl)ruthenium(II)/tri-n-butylphosphine, and bis(eta 5-cyclooctadienyl)ruthenium (II)/trialkylphosphine/maleic anhydride, ruthenium trichloride, ruthenium dicarbonyl bis-triphenylphosphine acetate dimer, and ruthenium tricarbonyl bis-triphenylphosphine.
The major by-product in the ruthenium catalyzed reaction of acetylenically unsaturated compounds with carboxylic acids is the carboxylic acid anhydride. In many cases, the rate of anhydride formation is equal to or greater than the rate of-vinyl ester formation.
Attempts to control by-product formation include the use of phosphine ligands to inhibit anhydride formation and increase vinyl ester selectivity, and removal of the vinyl ester product.
Another problem associated with the use of ruthenium catalyst in these reactions is catalyst stability. In general, catalyst stability is moderate and is dependent on the presence of ruthenium/phosphine ligand complexes. Two potential routes of deactivation include phosphine ligand oxidation and ruthenium cluster formation.
The use of ruthenium compositions as vinylation catalysts has overcome several of the deficiencies of the prior art, however; the need exists for a homogeneous liquid phase process for the production of vinyl esters, which provides higher selectivity at lower temperatures; with minimal catalyst degradation.